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Giving Earth That Worn-Down Feeling

We’ve all heard about how modern farming isn’t good for the land. Tilling and other practices leave farmland vulnerable to erosion; precious topsoil is literally being washed away.

Over the years, the issue has been framed in many ways. A 1984 NBC News report, for instance, described the amount of topsoil lost to erosion as enough to fill the Houston Astrodome 34,000 times over. More straightforward estimates suggest that worldwide, some 25 billion tons of valuable agricultural soil is eroded every year.

A study in Nature Geoscience puts the problem in another way, one that perhaps makes the situation seem even more dire. Conventional farming, the study’s authors write, erodes farmland at rates comparable to what nature does in some of the most rugged parts of the planet.

“We’re discovering that we as humans, in our agricultural practices, are actually eroding the land at a rate similar to the biggest glaciers and rivers in the world,” said the study’s lead author, Michelle N. Koppes of the University of British Columbia. “Humans are as powerful an agent of geomorphic change.”

So what’s wrong with that? The real problem, as Dr. Koppes notes, is that the land that humans are washing away is not being remade.

In a telephone interview from India, where she was about to visit some Himalayan glaciers, Dr. Koppes noted that the highest rates of erosion by rivers and glaciers occur in areas of tectonic uplift — where the land is still rising. So although erosion rates are high, there is always more mountain to erode.

But most farming takes place in lowland areas that are pretty quiet, tectonically speaking. “We’re doing work on landscapes where there isn’t the same rate of uplift,” Dr. Koppes said. “We’re denuding the land at a rate that isn’t sustainable.”

That point has also been made before, many times, but it helps to hear it from a different perspective. Dr. Koppes is a glacial geomorphologist whose main interest these days is how glaciers respond to climate change; her co-author, David R. Montgomery of the University of Washington, is a geomorphologist, too, but a fluvial one — he comes at the subject from the river side.

Their study wasn’t even mostly about comparing the effects of people to those of nature. Their main conclusion is that rivers and glaciers erode land at roughly the same rates. That finding may be of more interest to geomorphologists than anyone else, but it is important in its own way, as it flies in the face of much previous thinking.

Scientists had long thought that glaciers were more effective agents of erosion, and, given the ruggedness of much glaciated terrain — Glacier National Park comes to mind — it’s not hard to see why they might have thought this. In the mid 1990s their thinking was bolstered by a couple of studies that concluded that for basins of the same size, glaciers eroded the landscape up to 10 times faster than rivers.

But since then, techniques for studying sediment — particularly for learning what eroded when — have improved, as have methods for modeling the processes involved. Dr. Koppes and Dr. Montgomery compiled data on erosion rates from glaciated and non-glaciated areas around the world. They found that in a tectonically active area like Taiwan, erosion rates due to rivers were about the same as the rates due to glaciers in other tectonically active places like Alaska and Patagonia. “That goes against what was the conventional wisdom,” Dr. Koppes said.

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By 2050 or so, the human population is expected to pass nine billion. Those billions will be seeking food, water and other resources on a planet where humans are already shaping climate and the web of life. Dot Earth was created by Andrew Revkin in October 2007 -- in part with support from a John Simon Guggenheim Fellowship -- to explore ways to balance human needs and the planet's limits.